Embodiments according to the invention relate to a voltage converter circuit for a clocked supply of energy to an energy storage based on an input voltage. Such a voltage converter circuit may, for example, be used in connection with an up-conversion of an output voltage of an energy source, like, e.g., that of a thermal generator, a fuel cell or a solar cell. Further, embodiments according to the present invention relate to a method for the clocked supply of energy to an energy storage.
The technical field of use of embodiments according to the invention may, for example, be a voltage converter circuit which may already convert this voltage with regard to its voltage value at a low or lower input voltage, respectively. Embodiments according to the invention may be an up-voltage converter with coupled coils which provides at its output a higher voltage than the one applied to its input. Here, the voltage converter may be coupled to energy sources, like, e.g., thermal generators or solar cells, which only provide a very low output voltage as an input voltage for the voltage converter. By the use of an inventive voltage converter circuit, with still lower dimensions of the voltage converter, high efficiency may be achieved. The technical field of use may, as it is illustrated in further embodiments of the invention, be the setup of a self-oscillating voltage converter, which may start at operating voltages below a threshold voltage of a switching transistor of the voltage converter and still provide efficiencies of more than 70% with comparative circuit dimensions. Conventional, integrated DC/DC up-converters may have a low efficiency if, for example, they work with input voltages below 1 V.
Conventional DC/DC up-converters are available as integrated circuits which operate starting from an input voltage of approx. 300 mV. The efficiency of these DC/DC up-converters is usually low, however. DC/DC converters which are used in conventional devices like, e.g., mobile phones, laptops, etc., are, except for at least one external coil, available fully integrated on a chip. They offer high efficiencies with input voltages above 1.8 V. Input voltages below that, however, cause a rapid decrease of the efficiency here. This means that energy sources, like, e.g., solar cells and thermal generators, have to be connected cascadedly to provide a usable voltage for a DC/DC converter or up-converter, respectively. In particular with the thermal generators, this is only possible restrictedly, if the dimensions of the overall system are to be kept low. There is thus a need for a voltage converter circuit for energy sources which provide only a very low output voltage, wherein this low output voltage may be converted with a high efficiency into a changed output voltage of the voltage converter circuit, having at the same time low dimensions.
A self-oscillating voltage converter which can do without an active circuit for controlling the switching transistor for the voltage conversion and builds up oscillation itself may, for example, operate at 300 mV, but for building up oscillation for the circuit a clearly higher input voltage is requested. This starting voltage conventionally directly depends on the threshold voltage of the switching transistor, as the switching transistor may not supply current requested for startup below that. The starting voltage, apart from that, directly depends on the turns ratio of the transformer of the voltage converter circuit, which is formed from the coupling of the transformer coils. If a voltage converter circuit is to start as soon as possible, a high turns ratio in a range of, for example, 1:10 is needed. By this, when starting up, the low input voltage is up-transformed to clock the switching transistor through. When the desired output voltage has been achieved, the switching transistor at its control (gate) terminal is virtually stressed with the output voltage of the converter multiplied by the turns ratio, i.e., for example, 20 V, with an output voltage of 2 V and a turns ratio of 1:10. These occurring control (gate) voltages are not needed in a static operation for switching the transistor and lead to high switching losses.